1. Field of the Invention
The present invention relates to a composite retarder plate and an optically compensatory polarizing plate which can highly compensate for birefringence owing to TN liquid crystal in order to form a liquid-crystal display device excellent in viewing angle and contrast.
The present application is based on Japanese Patent Application No. Hei. 11-269059, which is incorporated herein by reference.
2. Description of the Related Art
TFT-LCD (liquid-crystal display) devices using TN liquid crystal have come into wide use for television sets, personal computer monitors, etc. because quick response characteristic and frontal-view high contrast characteristic have attracted a great deal of attention. In the meantime, the viewing angle range permitting good visibility has been narrowed owing to remarkable lowering of oblique-view contrast, reversal of gradation display (tone reversal), etc. The viewing angle range has needed to be mended. Improvement of contrast, widening of the viewing angle range, suppression of display color change owing to viewing angle and uniformity of screen display have become particularly important subjects with the advance of increase in screen size.
A proposal to use a retarder for compensating for phase difference owing to birefringence of TN liquid crystal in order to widen the viewing angle has been made heretofore as a measure against the aforementioned narrowed viewing angle range. A Wide View Film (tradename: made by Fuji Photo Film Co., Ltd.), an NH Film (tradename: made by Nippon Petrochemicals Co., Ltd.) and superposed compensating plates (Unexamined Japanese Patent Publication No. Hei. 7-306406, Unexamined Japanese Patent Publication No. Hei. 7-35924 and Unexamined Japanese Patent Publication No. Hei. 10-123506) are known as compensating plates for widening the viewing angle. The. Wide View Film is made of discotic liquid crystal with an inclined optical axis and exhibits negative refractive index anisotropy. The NH Film is made of nematic liquid crystal with an inclined optical axis and exhibits positive refractive in ex anisotropy. Each of the superposed compensating plates is made of a laminate of retarders. Each of the retarders is made of a uniaxial oriented film of a polymer exhibiting birefringence characteristic. In each of the superposed compensating plates, a retarder having an optical axis in a plane and a retarder having an optical axis inclined with respect to the plane are used in combination so that refractive index directions of the retarders cross at right angles.
The Wide View Film, however, had a problem that a coloring phenomenon occurred in a white display state without application of any voltage because of remarkable lowering of contrast at a viewing angle inclined by 60 degrees or more with respect to the frontal direction. The NH Film had a problem that a black display state with application of a voltage was not kept when the viewing angle was changed in the black display state. The superposed compensating plates had a problem that a remarkable coloring phenomenon occurred owing to the change of the viewing angle. Hence, the background-art compensating plates could not be adapted to phase difference characteristic of TN liquid crystal satisfactorily, and there was a problem that improvement in visibility characteristic of the background-art compensating plates was not sufficient.